Why is my vault's bevel unevenly applied?
How do you get to Killarney?  Well, sir, I wouldn't start from here if
  I were you....


Just teasing.. I think some of this may have been be easier if executed before the array was applied. (Making an array out of arch-peak-to-arch-peak sections may still speed things up when adding detail?) This is a fix, rather than a solution, I'd like to know why the Bevel works the way it does.. whether the algorithm could be improved.


Make a copy of your existing path-curves (you can CtrlJ join them into one curve object to save time later) call it, say 'Miter Path'
Make a copy of your existing profile curve, and delete all but its flat base .. 'Molding Base'
Use the Molding Base as a the Bevel Object for your Miter Path. You now have a set of ribbons
With the ribbons anywhere you like in Z, select them, and then the Vault object. Put the (active) Vault into Edit Mode
From an orthogonal view down Z, hit 'Knife Project' in the Tool region. This will cut a set of edges into the vault where you would have liked the bevels to be.
There will be some extraneous vertices.. there are various ways of tidying them up ..  I selected one of them , then used Select > Similar > Number of Connecting Edges to find the rest, and then X > Limited Dissolve to get rid of them.
I also selected the penultimate vertices around the pole of the vault in order, hit J to make a ring of edges, and deleted the faces at the pole, assuming there will be some kind of boss or rosette there in the end.
Now you can delete the faces between the knife-projected edges and CtrlE > Bridge Edge loops creating the flat bevel to accommodate the molding. CtrlR cut a loop down the middle of each bevel.
ShiftD Duplicate the central loops of the bevel, 
P separate to a new object, and AltC convert to a curve - U resolution 1 - this will be your new path for the molding profile
Move the origin of your molding profile to the center of its base - (this point will lie on the path). Use the molding profile as the Bevel Object for your new Molding Path. You may need to rotate it in Z in Edit Mode to get the molding to face the right way.


I apologize  for the lack of step-by step illustrations - everything important is done by step 5.





Alterations on Layer 11.